Research On The Luminescence Properties Of Transition Metal Chromium Ions Activated Borate Near Infrared Phosphors

Near-infrared phosphor-converted LEDs（NIR pc-LEDs）are expected to become the next-generation intelligent near-infrared light source,widely used in fields such as near-infrared spectral analysis,plant lighting,security monitoring,food testing,and biological imaging.The near-infrared fluorescence phosphor is an important component of NIR pc-LEDs,which directly determines the spectral range and electro-optical conversion efficiency of the device.The transition metal Cr³⁺ion-activated luminescent material is currently the preferred solution for near-infrared fluorescence phosphors.Cr³⁺ions have a 3d³ outer electron structure and exhibit intra-shell transitions,and by adjusting the crystal field intensity,the emission from deep red to near-infrared can be achieved.Currently,research on Cr³⁺ion-activated near-infrared fluorescence phosphors mainly focuses on spectral extension,improving luminescent efficiency,and thermal stability.In addition,to further improve the optoelectronic performance of packaging devices and meet the needs of the flourishing Mini/Micro LED technology and the biological field,the development of efficient and stable small-size near-infrared fluorescence phosphors is also urgent.In this paper,Cr³⁺ions were used as the activator ion and borate was used as the matrix to develop near-infrared fluorescence phosphors with different emission wavelengths,and the luminescence characteristics and mechanism were analyzed in depth.A new strategy for improving absorption,luminescent efficiency,and thermal stability was proposed,and high-quality synthesis technology for nano-scale near-infrared fluorescence phosphors was explored.The specific research content and results are as follows:1、YAl₃（BO）₄:Cr³⁺micrometer near-infrared fluorescence phosphors were synthesized by high-temperature solid-phase method,which can be excited by blue light and emit from the ²E and ⁴T₂ levels of Cr³⁺ions.The emission spectrum covers 700-950 nm,with a center wavelength of 730 nm,and the emission spectrum can perfectly match the absorption of plant photoreceptor P_FR.Meanwhile,YAl₃（BO）₄:0.12Cr³⁺exhibits high quantum efficiency and excellent thermal stability,with an internal quantum efficiency of up to 86.7%and almost zero thermal quenching,maintaining about 99%of room temperature luminescence intensity at150^oC.By using temperature-dependent spectroscopy/fluorescence lifetime testing and crystal field analysis,the anomalous thermal enhancement is due to the thermal coupling of ⁴T₂ and ²E levels.Near-infrared pc-LED devices were fabricated by combining YAl₃（BO）₄:0.12Cr³⁺phosphors with blue LED chips.At a driving current of 100 m A,the output power of the prepared pc-LED device was 50.6 m W,and the photoelectric conversion efficiency was 17.4%.The produced pc-LED device has high photoelectric conversion efficiency,and the output spectrum is highly compatible with the absorption of plant P_FR,which is expected to be used in the field of plant lighting.2、Based on the unique luminescence characteristics of YAl₃（BO）₄:Cr³⁺phosphors,the application of these phosphors in the field of temperature sensing was explored,and the temperature sensing sensitivity was regulated by replacing rare earth ions.A series of Ln Al₃（BO）₄:Cr³⁺（Ln=Gd,Y,and Lu）micro-near-infrared phosphors were synthesized by high-temperature solid-state method,and their crystal structure,luminescence characteristics,and the influence of Ln ion types on the luminescence of Cr³⁺were analyzed.The micro-near-infrared phosphors of Ln Al₃（BO）₄:Cr³⁺exhibit the ⁴T₂ and ²E level emissions of Cr³⁺ions,and the ratio of ⁴T₂ and ²E emission intensities(FIR(I₇₃₀/I₆₈₃))varies with temperature regulation,making it suitable for use in the field of temperature sensing.Furthermore,the energy difference between the ⁴T₂ and ²E levels of Cr³⁺ions can be adjusted by changing the Ln ion type,and the energy difference between the ⁴T₂ and ²E levels can be increased,thereby further improving the relative sensitivity.Based on the crystal structure and spectral analysis,with the decreased of Ln³⁺ions radius,the crystal field strength occupied by Cr³⁺ions is enhanced in the[Al O₆]octahedron,thereby increasing the energy difference between the ⁴T₂ and ²E levels and improving the relative sensitivity.3、High-performance submicron-sized In BO₃:Cr³⁺near-infrared phosphors were successfully prepared using a molten salt method.Boric acid was used as the molten salt to reduce the grain size and avoid the introduction of heteroatoms,resulting in the successful synthesis of spherical,submicron-sized（1μm）,smooth-surfaced,and relatively uniform-distributed In BO₃:Cr³⁺near-infrared phosphors.Under blue light excitation,the emission center of the submicron-sized In BO₃:x Cr³⁺near-infrared phosphors is located at 820 nm,and the spectral range is 700-1100 nm,with a half-width of 140 nm.Its quantum efficiency is 62%,which is superior to that of micro-sized In BO₃:Cr³⁺synthesized by high-temperature solid-state method（2-3μm;IQY～46%）.Further improvement in the absorption efficiency of submicron-sized In BO₃:Cr³⁺was achieved through co-doping with La³⁺ions,increasing from 15%(In BO₃:0.02Cr³⁺)to 27%(In BO₃:0.02Cr³⁺,0.04La³⁺).The absorption enhancement mechanism of In BO₃:Cr³⁺,La³⁺was systematically analyzed,and the results showed that the increase in absorption was due to the distortion of the[In O₆]octahedron induced by La³⁺ions,which caused a change in the symmetry of the octahedral crystal field,breaking the parity-forbidden transition restriction and achieving absorption enhancement.Co-doping with Yb³⁺ions improved the luminescence efficiency and thermal stability of submicron-sized In BO₃:Cr³⁺,and the internal quantum efficiency was increased to 73.7%(In BO₃:0.02Cr³⁺,0.005Yb³⁺),and the room temperature emission intensity could be maintained at 96%at 100°C(In BO₃:0.02Cr³⁺,0.03Yb³⁺).A deep analysis of the emission enhancement mechanism of In BO₃:Cr³⁺,Yb³⁺has been carried out,and the research indicates that the improvement in luminescence performance is due to the energy transfer from Cr³⁺to Yb³⁺.The excited energy from Cr³⁺ions is transferred to the more efficient and stable luminescent center of Yb³⁺ions.4、Nanoscale In BO₃:Cr³⁺near-infrared fluorescent powder was prepared by the molten salt-isolation method.Boric acid and silica were used as the molten salt and isolation medium to successfully reduce particle size.By optimizing the ratio of isolation medium Si O₂,raw material mixing uniformity,and temperature regime,spherical,nanosized（～120 nm）,smooth-surfaced,and relatively evenly distributed nanoscale In BO₃:Cr³⁺near-infrared fluorescent powder was synthesized.Compared with submicron In BO₃:Cr³⁺near-infrared fluorescent powder,its surface-to-volume ratio was increased by 8 times,and the basic luminescence characteristics remained unchanged,but the internal quantum efficiency and thermal stability were decreased.The internal quantum efficiency can reach 40%,and the luminescence intensity at room temperature can be maintained at 45%under 150°C,which is similar to the luminescent performance of micron-scale In BO₃:Cr³⁺synthesized by the high-temperature solid-phase method.Then,Yb³⁺ions were introduced to broaden the spectrum and improve luminescence performance,achieving long-wave broadband emission from 700 to 1200 nm,which can meet the requirements of food testing and biological fields.By adjusting the doping concentration of Cr³⁺and Yb³⁺ions,the energy transfer efficiency from Cr³⁺to Yb³⁺can reach 81.4%,and zero thermal quenching was achieved at 100°C.In the nanoscale In BO₃:0.005Cr³⁺,0.005Yb³⁺near-infrared fluorescent powder,the emission of Cr³⁺ions was weak,with an internal quantum efficiency of 67.8%,and the luminescence intensity at room temperature can be maintained at76%under 150°C.This result indicates that Cr³⁺ions can rapidly transfer energy to Yb³⁺ions,achieving more efficient and stable short-wavelength infrared emission of Yb³⁺ions under blue light excitation,which provides feasibility for the construction of miniaturized NIR pc-LED devices.